Cu clad stainless or carbon steels have been used in self-brazing applications in which a large number of joints, such as in the heat exchangers, is required. The self-brazing alloys with Cu cladding provide good wetting during the brazing and eliminate the use of flux. By roll bonding brazing alloys to a suitable base metal and forming a composite layer, the materials can meet a multitude of requirements simultaneously. Examples of these requirements are mechanical strength, chemical resistance, and heat transfer. Meanwhile, the self-brazing approach reduces part complexity and ensures a tight fit. Commercially available self-brazing alloys for the heat exchanger applications have been designed using commercially pure Cu as the brazing layer with base alloys of stainless and carbon steels selected for the mechanical strength and corrosion resistance. The useful upper temperature of brazed assembly produced from the conventional self-brazing materials is thus limited by the oxidation resistance of the Cu alloys.
Currently, elevated temperature heat exchangers are mostly brazed with Ni-based brazing alloys, which are produced by the rapid solidification process to obtain the necessary thin gauge. The cost of rapidly solidified Ni brazing alloys is high, and the available gauge and alloy chemistry is limited. However, the oxidation resistance requirements of heat exchangers and honeycomb structures preclude the use of Cu alloys in the brazed assemblies. Thus self-brazing alloys, which can be used at elevated temperature and cost effective as well, are needed.
Al and Ni-bearing Cu alloys have shown superior oxidation resistance in comparison to commercially pure Cu alloys. The observation of reduced oxidation weight gain of Al and Ni-bearing Cu alloys prompted the idea to use these alloys as the self-brazing layers to replace the commonly used commercially pure Cu and as an alternative to Ni-based brazing alloys.
This invention describes self-brazing alloys for elevated temperature applications that are produced by the economical roll bonding process. In the roll bonding process, Al and Ni-bearing Cu alloy strips are clad to either one or both sides of suitable base alloys. (There is no need for the application of flux, nor for its subsequent removal to produce the brazed assemblies. The initial purchase cost of flux, as well as the cost associated with the disposal of cleaning by-product, can be largely eliminated.) The Al and Ni-bearing, self-brazing layer can be either Cu alloys containing the necessary Al and Ni addition or composites of Cu/Al or Cu/Ni. Furthermore, the composites are not limited to Cu/Al or Cu/Ni, but can also be, for example, Cu/Al/Cu or Cu/Ni/Cu.